Wide dynamic range image sensor

ABSTRACT

An exemplary image sensor comprises a photodetector proximate to a pixel site, and a light meter proximate to the pixel site configured to approximate an initial charge acquired by the photodetector at the end of a first integration period of a frame exposure period. A reset circuit resets the photodetector if the approximated initial charge acquired by the photodetector exceeds a threshold. A readout circuit detects a final charge acquired by the photodetector at the end of a second integration period of the frame exposure period. If the photodetector was reset, the readout circuit adjusts the final exposure to account for exposure prior to the photodetector having been reset.

This is a continuation of application Ser. No. 11/592,416, Filed on Nov.3, 2006.

The present application claims the benefit of and priority to a pendingprovisional patent application entitled “Wide Dynamic Range CMOS ImageSensor Circuit and Method of Use,” Ser. No. 60/800,129 filed on May 12,2006. The disclosure in that pending provisional application is herebyincorporated fully by reference into the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of imaging devices. Morespecifically, the invention is in the field of imaging devices havingimproved dynamic range.

2. Background Art

Image sensors have broad applications in many areas. Image sensorsconvert a received image into representative information indicative ofthe received image. Examples of image sensors include solid-state imagesensors, such as charge coupled devices (CCDs) and CMOS imaging devices(also known as CMOS image sensors), among others.

Image sensors are fabricated from semiconductor materials and compriseimaging arrays of light detecting, i.e., photosensitive, elements (alsoknown as photodetectors) interconnected to generate representativeinformation (e.g., analog signals) that corresponds to an imageilluminating the device. A typical imaging array comprises a number ofphotodetectors arranged in a pattern, for example, a pattern comprisingrows and columns. Each photodetector in the imaging array receives aportion of the light reflected from an object and received by the imagesensor. Each portion is called a picture element and is typicallyreferred to as a pixel. Each pixel provides output informationrepresentative of the luminance and/or chrominance detected by thephotodetector. When considered in the context of the pattern of thephotodetectors, the output information from the pixels constitutes therepresentative information that corresponds to the image incident uponthe imaging array.

As an example, each pixel of an imaging array may provide as outputinformation an output signal, which may, for example, be dependent uponan accumulation of charge, i.e., photo charges, according to thephotoelectric effect, corresponding to the radiation intensity fallingupon its detecting area defined by the physical dimensions of itscorresponding photodetector. The photo charges from each pixel areconverted to a charge signal, which is an electrical potentialrepresentative of the luminance and/or chrominance reflected from arespective portion of the object and received by the image sensor. Theresulting charge signal or potential is read and processed byvideo/image processing circuitry to create a signal representation ofthe image.

It has been difficult to provide simultaneously wide dynamic range andvery low noise in image sensor arrays. Present trends towards smallpixel pitches in CMOS four-transistor (4T) technology result in loss ofcharge capacity as the pixel shrinks. This results in imagers with only56 dB or less of dynamic range. This low dynamic range degrades thequality of outdoor images because there is simply not enough range inthe sensor photodiode to describe both the bright and dark areas of thescene.

A conventional CMOS image sensor is typically structured as an imagingarray of photodetectors, each photodetector being reset to anapproximately known potential after the readout of a previous image, andin preparation for the next image. However, the performance ofconventional CMOS image sensors suffers from a number of problems. Forexample, conventional CMOS image sensors suffer from noise associatedwith the process for resetting the photodiode in each pixel to a knownpotential after each exposure and in preparation for the next image.This noise, also referred to as reset noise or KTC noise, is often asignificant source of noise in camera systems employing conventionalCMOS image sensors. The reset noise is proportional to the square rootof KTC, where C is the capacitance of the sense node orphotodiode/source follower gate combination in a typical active pixelsensor. Reset noise is typically 30 to 40 electrons one-sigma. Reducingthe capacitance of the sense node can reduce reset noise, but results ina corresponding reduction in the total charge that can be collected andtherefore undesirably reduces the overall dynamic range in the camerasystem.

Moreover, the trend towards small pixel pitches in image sensors resultsin additional loss of charge capacity, further reducing dynamic range.Low dynamic range significantly degrades image quality, particularly inoutdoor images, due to the limited range of the sensor's photodetectorand its inability to describe both bright and dark areas of the scene ina single frame exposure.

One approach for increasing dynamic range in image sensors involvesacquiring both a short exposure and a long exposure separately, eachexposure stored in respective frame buffers, the short exposure beingsuitable for capturing bright areas of a scene, and the long exposurebeing suitable for capturing the dark areas of the scene. Thereafter,the image stored into the short exposure frame buffer and the imagestored into the long exposure frame buffer could be integrated into asingle image with improved dynamic range. However, the significantlyincreased costs associated with additional frame buffers renders thisapproach impractical for many applications.

Three-transistor (3T) pixels can be constructed with large capacities(e.g., 15,000 to 35,000 photons or more), but the higher noise floor of25 to 35 electrons (converted photons) results in a dynamic range of 60dB at best. However, in low light situations, the higher noise floor andhigher dark current of the 3T pixel result in greatly degradedperformance compared to 4T pixels.

In higher cost systems, such as those used for security applications,very high dynamic range images can be compounded by collecting multiplefull-frame images and different exposures. These outputs are stored in afull-frame memory buffer and a very high dynamic image is created bysoftware. However, the required buffer memory cost is comparable to thecost of the image sensor, and this cost is not easily borne by low-endconsumer applications, such as cellular phone cameras. However, inhigher end camera phones, substantial improvements in overallperformance can be enabled by the use of a frame buffer in the camera.

Four-transistor (4T) pixels with pitches of about 2.25 micron to 2.8micron struggle to have 6000 electrons of capacity while maintaining anoise floor of about 10 electrons. This lower noise floor results andthe lack of significant dark signal noise results in three to four timesimprovement in low light performance when compared to 3T technology.However, the total dynamic range remains at 56 dB, and outdoor imagesare inferior to larger pixel cameras.

What is needed is a low cost solution that retains the low lightperformance and small form factor of the small pixel pitch but has asubstantial (e.g., 2×) improvement in dynamic range for outdoor images.Accordingly, there is a strong need in the art for a cost-effective widedynamic range image sensor.

SUMMARY OF THE INVENTION

The present invention is directed to a wide dynamic range image sensorand method of use. In one exemplary embodiment, the image sensorcomprises a photodetector proximate to a pixel site, and a light meterproximate to the pixel site configured to approximate an initial chargeacquired by the photodetector at the end of a first integration periodof a frame exposure period. For example, the first integration periodmay be about half of the frame exposure period. In one embodiment, aplurality of photodetectors are provided at a pixel site of the imagesensor, such that a first photodetector, a second photodetector, a thirdphotodetector, and a fourth photodetector are in a Bayer pattern. ABayer pattern is an arrangement of photodetectors having varyingsensitivity to luminance and chrominance in proximity to one another.For example, a rectangular area may be divided into four quadrants, withone quadrant configured as a red-sensitive photodetector, one quadrantconfigured as a blue-sensitive photodetector, and two (preferablydiagonally opposite) quadrants configured as green-sensitivephotodetectors.

A reset circuit resets the photodetector if the approximated initialexposure of (e.g., charge acquired by) the photodetector at the end of afirst integration period of a frame exposure period exceeds a threshold.A readout circuit is configured to detect a final exposure of (e.g.,charge acquired by) the photodetector at the end of a second integrationperiod of the frame exposure period. If the photodetector was reset, thereadout circuit is configured to adjust the final exposure, for example,by increasing the final exposure by a predetermined factor, such as by afactor of two. In one embodiment, the light meter comprises asupplemental photodetector proximate to the pixel site. The supplementalphotodetector may also be configured to store information indicative ofthe photodetector having been reset. In another embodiment, a sense nodecoupled to the photodetector through a transfer transistor functions asa light meter.

As described more fully below, an innovative image sensor is providedthat has significantly wider dynamic range, yielding a more accurateimage acquisition operation, ultimately resulting in an improved finalimage quality. Other features and advantages of the present inventionwill become more readily apparent to those of ordinary skill in the artafter reviewing the following detailed description and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary image sensor circuit according to oneembodiment of the present invention.

FIG. 2 illustrates a flow chart for operating the image sensor circuitof FIG. 1 according to one embodiment of the present invention.

FIG. 3 illustrates an exemplary pixel site arranged in a Bayer patternaccording to one embodiment of the present invention.

FIG. 4 is a block diagram illustrating an apparatus in accordance withat least one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a wide dynamic range image sensorand method of use. The following description contains specificinformation pertaining to the implementation of the present invention.One skilled in the art will recognize that the present invention may beimplemented in a manner different from that specifically discussed inthe present application. Moreover, some of the specific details of theinvention are not discussed in order to not obscure the invention. Thespecific details not described in the present application are within theknowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely exemplary embodiments of theinvention. To maintain brevity, other embodiments of the invention whichuse the principles of the present invention are not specificallydescribed in the present application and are not specificallyillustrated by the present drawings. It is noted that, for ease ofillustration, the various elements and dimensions shown in the drawingsare not drawn to scale.

By way of overview, according to the present invention, a camera system(e.g., a CMOS camera system) can be adapted to capture images with twointegration times for a single frame. The supporting or integrated pixelprocessing chain combines the two embedded images into a single imagewith a total dynamic range which can be approximately equivalent to adynamic range of a pixel with a photon capacity that is double that ofthe actual pixel. For example, a typical 4T pixel with 6000 electroncapacity would render an image equivalent to 12000 electron capacity fora maximum dynamic range of 62 dB, providing an improvement of 6 dB.

Such an increase in dynamic range can be accomplished through the use ofa novel pixel and readout approach for shared pixels. Image informationis collected by four pixels using a common sense node. During outdoorexposures an overall integration time for the frame that is longer thanthat which the pixel capacity will support is selected. Withoutadditional action this would result in only the image features in shadowbeing correctly exposed, while the image features in bright light (e.g.,sunlight) would be overexposed.

After some portion of time during the exposure less than that needed forthe brightest pixels to reach capacity, a rapid read operation isperformed to examine the amount of exposure of a pixel. Such a rapidread operation may be performed by reading an extra pixel element. Theextra pixel element may be either a fifth pixel element explicitlyfabricated for use as a light meter or an existing circuit feature thatexhibits a photosensitive property. The preferred method is to read afifth pixel element.

The fifth pixel element can be, for example, a conventional 3Tphotodetector which is associated with a shared cluster of four pixels,where each pixel can be, for example, a 4T pixel. The fifth pixelelement can be read and reset. The fifth pixel element may be a verysmall 3T pixel as there are no constraints or concerns about pixel fixedpattern noise. The light response level of the fifth pixel element canbe tuned by using light shields to insure it is not too responsive. The3T fifth pixel element is reset to a high voltage at the start of frameintegration. After the completion of a portion (e.g., one half) of thetotal integration time, the fifth pixel element is read. This rapid readout may be accomplished by a column analog-to-digital-converter (ADC)comparator, and each pixel in each row is compared to a preset value.Pixels whose sense node voltage has decreased below the preset valuehave been exposed to bright light and would become over filled anduseless to the final image if integration were to continue to the fullextended exposure time (e.g., twice the normal exposure time). When thecolumn ADC recognizes the depleted condition (i.e., exposure to brightlight) of the sense node in the fifth pixel element for this sharedpixel cluster of four 4T photodiodes, a logic latch is set in the columnread out circuits. This depleted state signal is thus recorded on aline-by-line basis. This column stored information is used to then resetthe each depleted pixel in this row immediately subsequent to the sensenode read operation. A record of this reset event may be stored in thepixel by writing or resetting the fifth pixel element to a recognizablevalue, such as a very low voltage.

Another method is to poll or read the sense node. The shared sense nodeis exposed to light, and its voltage drops in proportion to the lightcollection in the local area. The sense node voltage will drop rapidlywhen the local pixels become over saturated. The transfer gates for allfour regular photodetectors remain closed during this read operation andonly the charge remaining on the common sense node is polled. This rapidread out is accomplished by a column ADC comparator, and each pixel ineach row is compared to a preset value. Pixels whose sense node voltagehas decreased below the preset value have been exposed to bright lightand would become over filled and useless to the final image ifintegration were to continue for the full exposure duration (e.g., twicethe normal exposure duration). When the column ADC recognizes thedepleted condition of the sense node for these shared pixel clusters inbright light, a logic latch is set in the column read out circuits. Thisdepleted state signal is thus recorded on a line-by-line basis. Thiscolumn stored information is used to then reset the each depleted pixelin this row immediately subsequent to the sense node read operation. Arecord of this reset event may be stored in an element capable ofstoring state information, such as in the pixel by writing or resettingthe fifth pixel element to recognizable value, such as a very lowvoltage, or in a memory cell.

The pixel will contain a state information storage element, such as afifth pixel element or a memory cell that may be integrated with theimage sensor or in a separate memory device. The element capable ofstoring state information is preferably reset or state changed only atthe intermediate exposure point before the full extended exposure timefor the frame elapses. When the final image is read the state of thestate information storage element is also read. The fifth pixel elementis a small third row of pixels which is associated both physically andlogically with each Bayer pattern set of conventional pixels (e.g., oneconventional pixel or two conventional pixels). The state of the fifthpixel element tells the pixel processing chain that the associatedshared imaging pixels were reset at the intermediate exposure point,indicating that the values are to be scaled accordingly (e.g., 2×) inthe final image representation. (This state can be appended to the “bitsfor the pixel” as an added bit in the column circuits during read out.)The fifth pixel element is a simple diode storage node which is resethigh (or low, simple differential state) when the associated fourimaging pixels are selected for restart of integration at anintermediate time.

A second method to determine whether an intermediate reset was appliedis to again poll the shared sense node at the end of integration timefor the entire frame. For example, by determining the response of theshared sense node to illumination, appropriate thresholds may be set forcomparing the polling of the shared sense node to determine whether anintermediate reset was applied.

A rapid read of the fifth pixel element is used to compare to a fixedvalue before the frame exposure is complete. This is best and mosteasily accomplished by a column ADC approach. In addition, the columncircuits or a column mapped memory is used to remember the results, anda decision based on this sampling of the light meter pixel is used toinstruct which pixels should be reset to insure maximum dynamic range.Also, the vertical scanner module is adapted to accommodate theprovision of a variable reset voltage to each set of three rows. Anadditional dummy row is preferably added to the system to maintainbalance of analog loads. This dummy row is reset for all pixels columnaddress that are not reset in the active rows during the same resetoperation, thereby keeping loading on the analog circuits constant.

The ratio of the amount of area occupied by light meter photodetectorsto the amount of area occupied by imaging photodetectors can be selectedto optimize performance. As more area is devoted to light meterphotodetectors, the number of photodetectors can be increased, which canincrease the granularity with which information from the light meterphotodetectors can be applied to nearby imaging photodetectors. As lessarea is devoted to light meter photodetectors, a larger proportion ofthe area is occupied by imaging photodetectors, which can minimizealiasing.

Referring to FIG. 1, there is shown exemplary CMOS image sensor circuit100 including pixel site area 102 and light meter 104 in accordance withone embodiment of the present invention. CMOS image sensor circuit 100may be employed in imaging devices for use in a variety of applications.As discussed below, CMOS image sensor circuit 100 is configured toacquire an image with significantly improved dynamic range, resulting insignificantly improved image quality.

In FIG. 1, CMOS image sensor circuit 100 is configured to control andread photodetectors, such as photodiodes 122, 124, 126 and 128 of pixelsite area 102, although other types of photodetectors may also be usedwith the present invention. Each photodiode 122, 124, 126 and 128 isassociated with a corresponding pixel in an imaging array of an imagingdevice. For example, FIG. 3 illustrates pixel site area 302 sub-dividedand arranged in a Bayer pattern, including green element areas 322 and328, blue element area 324 and red element area 326. FIG. 3 alsoillustrates a light meter area 362, proximate to green element areas 322and 328, blue element area 324, and red element area 326.

In an embodiment, where pixel site area 102 of FIG. 1 is configured aspixel site area 302 of FIG. 3, photodiodes 122 and 128 can be used tocapture the image scene falling at pixel element areas 322 and 328,respectively, of Bayer patter 302. Likewise, photodiode 126 can be usedto capture the image scene falling at pixel element area 326, andphotodiode 128 can be used to capture the image scene falling at pixelelement area 328. Various demosaicing algorithms can be used tointerpolate a set of complete red, green, and blue values for each ofpixel element areas 322, 324, 326 and 328 as is known in the art.

Also, photo diode 162 can be used as a fifth photodetector to measurelight incident on light meter area 362. Since light meter area 362 islocated in proximity to green element areas 322 and 328, blue elementarea 324, and red element area 326, light incident on light meter area362 is also indicative of light incident on green element areas 322 and328, blue element area 324, and red element area 326. Thus, photodiode162 can be used to control exposure of green element areas 322 and 328,blue element area 324, and red element 326.

Photodiodes 122, 124, 126 and 128 are shown in FIG. 1 and describedherein for illustrative purposes only to describe pixel area 102, and atypical imaging array comprises a larger number of photodiodes andpixels. As shown in FIG. 1, each photodiode 122, 124, 126 and 128 isconnected across ground 140 and shared sense node 120 by respectivetransfer transistor 112, 114, 116 and 118.

Reset transistor 106, source follower transistor 108 and selecttransistor 110 provide control and readout of CMOS image sensor circuit100, as described more fully below. Each of transfer transistor 112,114, 116 and 118, reset transistor 106, source follower transistor 108and select transistor 110 can, for example, comprise an N-channel FET(NFET). The drain of reset transistor 106 is tied to the drain of selecttransistor 110 at node 134, and the source of reset transistor 106 isconnected to the gate of source follower transistor 108 at shared sensenode 120. The drain of source follower transistor 108 is connected tothe source of select transistor 110, and the source of source followertransistor 108 is coupled at node 136 to current source 130 and sampleand hold circuit 180 via column bus 138.

Light meter 104 comprises a three-transistor (3T) pixel arrangementincluding supplemental photodiode 162, although other types ofphotodetectors may also be used with the present invention. Light meter104 includes light meter reset transistor 166, light meter sourcefollower transistor 168 and light meter select transistor 170 to providecontrol and readout of light meter 104, as described more fully below.Each of light meter reset transistor 166, light meter source followertransistor 168 and light meter select transistor 170 can, for example,comprise an NFET.

Photodiode 162 is connected across ground 140 and the gate of lightmeter source follower transistor 168 at sense node 160. The source oflight meter reset transistor 166 is also tied to sense node 160, and thedrain of light meter reset transistor 166 is coupled at node 134 to thedrain of light meter source follower transistor 168. The drain of lightmeter select transistor 170 is tied to the source of light meter sourcefollower transistor 168, and the source of light meter select transistor170 is coupled at node 136 to current source 130.

A controller 190 provides control and timing signals to each of transfertransistors 112, 114, 116 and 118, reset transistor 106, source followertransistor 108, select transistor 110, light meter reset transistor 166,light meter source follower transistor 168 and light meter selecttransistor 170. More particularly, a cell high signal 152 is supplied tonode 134, a row reset signal 150 is supplied to the gate of resettransistor 106, a column reset enable signal 151 is supplied to the gateof column reset enable transistor 153, a row select signal 154 issupplied to the gate of select transistor 110, a transfer signal 1(Txfr1) 142 is supplied to the gate of transfer transistor 112, atransfer signal 2 (Txfr2) 144 is supplied to the gate of transfertransistor 114, a transfer signal 3 (Txfr3) 146 is supplied to the gateof transfer transistor 116, a transfer signal 4 (Txfr4) 148 is suppliedto the gate of transfer transistor 118, a meter select signal 172 issupplied to the gate of light meter select transistor 170, and a meterreset signal 174 is supplied to the gate of light meter reset transistor166.

In accordance with at least one embodiment, a controller is coupled tothe photodetector and to the light meter. The controller is configuredto cause an evaluation of an initial exposure of the photodetectorduring a first integration period of a frame exposure period. Thecontroller is further configured to cause a selective modification ofthe initial exposure of the photodetector prior to a second integrationperiod of the frame exposure period dependent upon the evaluation. Thecontroller is also configured to cause a final exposure of thephotodetector to be adjusted dependent upon the selective modification.

In at least one embodiment, the selective modification comprisesresetting the photodetector. In at least one embodiment, an indicationof the selective modification is stored in a memory cell. In at leastone embodiment, the light meter comprises a three transistorphotodetector circuit. In at least one embodiment, the light metercomprises a four transistor photodetector circuit. In at least oneembodiment, the light meter comprises a sense node coupled to thephotodetector via a transfer transistor.

In at least one embodiment, the controller is configured to cause asecond evaluation of a second exposure of the photodetector during thesecond integration period of the frame exposure period. The controlleris further configured to cause a second selective modification of thesecond exposure of the photodetector prior to a third integration periodof the frame exposure period dependent upon the second evaluation. Thecontroller is also configured to cause a final exposure of thephotodetector to be adjusted dependent upon the second selectivemodification.

Referring now to FIG. 2, an exemplary method for operating an imagesensor, such as CMOS image sensor circuit 100 of FIG. 1, will bedescribed in relation to flow chart 200. Flow chart 200 illustrates onetechnique for acquiring an image at pixel site area 102 of image sensorcircuit 100 of FIG. 1 during a frame exposure period. Certain detailshave been left out of flow chart 200 of FIG. 2 that are apparent to aperson of ordinary skill in the art. For example, a step may consist ofone or more sub-steps. While steps 202 through 218 shown in flow chart200 are sufficient to describe one embodiment of the present invention,other embodiments of the invention may utilize step different from thoseshown in flow chart 200. As described above, a typical imaging arraycomprises a larger number of photodiodes and pixels, and the techniqueillustrated in flow chart 200 and described below can be applied toprocessing a plurality of pixel areas, typically in rows of pixels.

At step 202, a command to begin an image capture is received, e.g., froma user, and the exposure process is initiated. In the method of flowchart 200, the frame exposure period is subdivided into a firstintegration period and a second integration period. In otherembodiments, additional integration periods can also be implemented toprovide finer granularity and possibly wider dynamic range. In thepresent example, the first integration period is approximately half ofthe exposure period. Typically, each of photodiodes 122, 124, 126 and128 and light meter photodiode 162 are reset to a known potential(reference charge level) prior to the initial integration period. Forexample, photodiodes 122, 124, 126 and 128 are reset by enabling resettransistor 106 and each of transfer transistors 112, 114, 126 and 128.Similarly, photodiode 162 is reset by activating light meter resettransistor 166.

At step 204, an initial charge acquired by photodiodes 122, 124, 126 and128 at pixel site area 102 are approximated at the end of the firstintegration period. Since 3T pixel light meter 104 is situated proximateto pixel site area 102, this initial charge approximation can bedetermined by detecting the charge acquired by photodiode 162 of lightmeter 104 at the end of the first integration period. In light meter104, the potential of photodiode 162 can be transferred to column bus138 and sample and hold circuit 180 by supplying meter select 172 to thegate of light meter select transistor 170 and activating light meterselect transistor 170.

Alternatively, the initial charge approximation can be determined bysampling the potential at shared sense node 120 since the interfacebetween sense node 120 and transistors 112, 114, 116 and 118 forms anN-P junction, effectively functioning as a photodiode, and usable as alight meter. In this particular embodiment, sense node 120 functions asa light meter, and 3T pixel light meter 104 can be omitted from CMOSimage circuit 100. The potential at shared sense node 120 can betransferred to column bus 138 and sample and hold circuit 180 bysupplying row select 154 to the gate of row select transistor 110 andactivating it to provide voltage/current to the drain of source followertransistor 108, such that transistor 108 can function as a sourcefollower and can transfer a voltage proportional to the voltage at thegate of transistor 108, i.e. the voltage at shared sense node 120, tocolumn bus 138.

The charge transferred from the light meter to column bus 138 is sampledand processed by sample and hold circuit 180 coupled to column bus 138to determine an approximate initial charge acquired by photodiodes 122,124, 126 and 128 at pixel site area 102. Since the light meter used toapproximate the initial charge acquired by photodiodes 122, 124, 126 and128 may be a photodetector having a capacity different from that ofphotodiodes 122, 124, 126 and 128, appropriate adjustment, mapping orscaling may be required in order to accurately correlate the light meterreading from the light meter with the actual initial charge acquired byphotodiodes 122, 124, 126 and 128.

At step 206, a determination is made as to whether the initial chargedetermined at step 204 exceeds a threshold. This threshold is typicallyexceeded in areas of a scene that are bright. Conversely, the thresholdis typically not exceeded in dark areas of a scene. For example, thethreshold may be selected such that the threshold with be exceededduring the first integration period if the capacity of thephotodetectors of the pixel would be exceeded, but for resetting thephotodetectors, during the frame exposure period. In the particularembodiment where the first integration period is approximately half ofthe exposure period, the threshold can be set to a value thatcorresponds with a determination that at least half of the chargecapacity of photodiodes 122, 124, 126 and 128 has been depleted as aresult of image acquisition during the first integration period. By wayof illustration, if each of photodiodes 122, 124, 126 and 128 has a10,000 electron charge capacity, the threshold can be set to an electroncharge value corresponding to approximately 5,000. In other embodiments,the threshold value can be set differently, such as a value thatcorresponds with a determination that at least one-third of the chargecapacity of photodiodes 122, 124, 126 and 128 has been consumed, forexample. If, at step 206, the initial charge capacity exceeds thethreshold, each of photodiodes 122, 124, 126 and 128 are reset at step208. If the initial charge capacity does not exceeds the threshold, flowchart 200 skips reset step 208, thereby accurately describing dark areasof a scene, and continues to step 212.

At step 208, photodiodes 122, 124, 126 and 128 are reset. The resetevent of step 208 provides a level of protection against over-saturationof photodiodes 122, 124, 126 and 128, e.g., when acquiring a bright areaof a scene. Since pixels are commonly reset in entire rows in a typicalimage sensor, a column reset enable transistor 153 operating as a switchmay be provided between row reset signal 150 and the gate of resettransistor 106 to selectively reset a particular column of a row duringthe reset event of step 208. In certain embodiments, row reset eventsare executed in pairs of rows, i.e., two rows at a time. In theseparticular embodiments, protection against analog device imbalance canbe provided during the reset event of step 208 by way of a placeholderor “dummy” row. Imbalances may occur as a result of differences in thenumber of pixels to be reset between two rows. Providing a dummy row asa placeholder element (to be reset) allows the two-row reset process tobe symmetrical and balanced despite the differences in the number ofpixels to be reset between two rows.

At step 210, the reset event of step 208 is recorded. In the embodimentdepicted in FIG. 1 employing 3T pixel light meter 104, the reset eventcan be stored by photodiode 162, e.g., by charging photodiode 162 to aknown voltage clearly indicative of a reset event. In the embodimentwhere sense node 120 functions as the light meter, the reset event canbe recorded in a storage latch.

At step 212, a final charge acquired by photodiodes 122, 124, 126 and128 at pixel site area 102 are determined at the end of the secondintegration period. In the example where the exposure period issub-divided into two integration periods (first and second), the end ofthe second integration period coincides with the end of the frameexposure period. In the embodiment shown in FIG. 1, this detection canbe carried out by activating row select transistor 110 and seriallytransferring the charge acquired by photodiodes 122, 124, 126 and 128 byserially selecting transfer transistors 112, 114, 126 and 128,respectively. The charge acquired by each of photodiodes 122, 124, 126and 128 are thereby serially transferred to shared sense node 120 andsubsequently processed along column bus 138.

At step 214, a determination is made as to whether photodiodes 122, 124,126 and 128 were reset at step 208. As described above, this reset eventis recorded at step 210 and is typically carried out for pixels inbright areas of a scene. If photodiodes 122, 124, 126 and 128 werereset, then step 216 is carried out; otherwise, flow chart 200 continuesto step 218.

At step 216, the final charge determined during step 212 is increased bya predetermined factor. For example, the value corresponding to thefinal charge capacity can be doubled. At step 218, the exposure processis completed.

This increase in the final charge during step 216 accounts for the resetevent of step 208. Advantageously, the technique described by flow chart200 avoids over-saturation of the photodetectors in bright areas of ascene and yet maintains accurate image acquisition of dark areas.Beneficially, a more accurate measure of the light acquired byphotodiodes 122, 124, 126 and 128 is achieved, and the dynamic range ofthe image sensor is effectively doubled without the high cost ofredundant image buffers.

FIG. 4 is a block diagram illustrating apparatus 400 in accordance withat least one embodiment. Apparatus 400 comprises a photodetector array415, column ADC circuits and control logic 441, storage latches 431,vertical scanner circuits 411, “dummy” row 420, and vertical scanner 421for “dummy” row 420. Photodetector array 415 comprises pixel sites, suchas pixel site 402, preferably arranged in Bayer patterns, as depicted inFIG. 3. In accordance with at least one embodiment, the pixel sitescomprise a light meter. For example, pixel site 402 is shown ascomprising light meter 462. Other examples of light meters for otherpixel sites include light meters 463 and 464. The elements in each rowof photodetector array 415 are duplicated in “dummy” row 420. Forexample, if pixel site 402 comprises light meter 462, “dummy” row 420would also comprise a “dummy” light meter, allowing all actions thatwould be undertaken with respect to photodetector array 415 to be ableto be undertaken with respect to “dummy” row 420. In accordance with atleast one embodiment, storage latches 431 can be used store informationindicating whether photodetectors were subject to an intermediate reset,in a manner described above in relation to FIGS. 1 and 2. Apparatus 400is thus one embodiment where the present invention as described in FIGS.1 and 2 above can be practiced.

From the above description of exemplary embodiments of the invention itis manifest that various techniques can be used for implementing theconcepts of the present invention without departing from its scope.Moreover, while the invention has been described with specific referenceto certain embodiments, a person of ordinary skill in the art wouldrecognize that changes could be made in form and detail withoutdeparting from the spirit and the scope of the invention. The describedexemplary embodiments are to be considered in all respects asillustrative and not restrictive. It should also be understood that theinvention is not limited to the particular exemplary embodimentsdescribed herein, but is capable of many rearrangements, modifications,and substitutions without departing from the scope of the invention.

Thus, a wide dynamic range image sensor and method of use have beendescribed.

1. An image sensor circuit comprising: a photodetector proximate to apixel site; a light meter proximate to said pixel site configured toapproximate initial exposure of said photodetector; a reset circuitconfigured to reset said photodetector if said initial exposure exceedsa threshold; a readout circuit configured to detect a final exposure ofsaid photodetector, said readout circuit further configured to adjustsaid final exposure if said photodetector was reset.
 2. The image sensorcircuit of claim 1 wherein said light meter is further configured toapproximate said initial exposure of aid photodetector at an end of afirst integration period of a frame exposure period.
 3. The image sensorcircuit of claim 1 wherein said readout circuit is further configured todetect said final exposure of said photodetector atm end of a secondintegration period of a frame exposure period.
 4. The image sensorcircuit of claim 1, wherein said light meter comprises a supplementalphotodetector proximate to said pixel site.
 5. The image sensor circuitof claim 1, wherein said supplemental photodetector is configured tostore information indicative of said photodetector having been reset. 6.The image sensor circuit of claim 1, wherein said light meter comprisesa sense node coupled to said photodetector.
 7. The image sensor circuitof claim 2, wherein said first integration period is approximately halfof said frame exposure period.
 8. The image sensor circuit of claim 1,wherein said photodetector comprises a plurality of photodetectorsincluding a first photodetector, a second photodetector, a thirdphotodetector, and a fourth photodetector, wherein said first, second,third and fourth photodetectors are arranged in a Bayer pattern.
 9. Anapparatus comprising: a photodetector proximate to a pixel site of animage sensor; a light meter proximate to said pixel site; a controllercoupled to said photodetector and to said light meter, said controllerconfigured to cause an evaluation of an initial exposure of saidphotodetector, said controller further configured to cause a selectivemodification of said initial exposure of said photodetector, saidcontroller further configured to cause a final exposure of thephotodetector to be adjusted dependent upon said selective modification.10. The apparatus of claim 9, wherein said controller is furtherconfigured to cause said evaluation of said initial exposure during afirst integration period of a frame exposure period.
 11. The apparatusof claim 9, wherein said controller is further configured to cause saidselective modification prior to a second integration period of saidframe exposure period dependent upon said evaluation.
 12. The apparatusof claim 9, wherein said selective modification comprises resetting saidphotodetector.
 13. The apparatus of claim 9, wherein an indication ofsaid selective modification is stored in a memory cell.
 14. Theapparatus of claim 9, wherein an indication of said selectivemodification is stored in said light meter.
 15. A method comprising:approximating an initial exposure of a plurality of photodetectorincluded in an image sensor, by a single reference common to saidplurality of photodetectors, at an end of a first integration period ofa frame exposure period; resetting said plurality of photodetectors ifsaid initial exposure exceeds a threshold; detecting a final exposure ofsaid plurality of photodetectors at an end of a second integrationperiod of said frame exposure period; adjusting said final exposure ifsaid plurality of photodetectors was reset.
 16. The method of claim 15,wherein said approximating said initial exposure further comprisesreading a sense node common to of photodetectors, said sense nodecoupled to said of photodetector through a plurality of respectivetransfer transistors.
 17. The method of claim 15, wherein said firstintegration period is approximately half of said frame exposure period.18. The method of claim 15, wherein said plurality of photodetectorsincludes a first photodetector, a second photodetector, a thirdphotodetector, and a fourth photodetector, wherein said first, second,third and fourth photodetectors are arranged in a Bayer pattern.